Recently, a transition has been made to using microwaves to heat the plastic preforms, in addition to or instead of infrared radiation. The microwave heating devices known in the prior art usually have a plurality of heating stations which are units that are each independent of one another. In order to heat the plastic preforms by means of microwaves, usually several individual process stations are operated in parallel, wherein these individual process stations are of identical construction and can also be operated independently of one another. Several apparatus parts are provided for generating the microwave radiation, such as in particular a high voltage generator, a magnetron, a waveguide for transmitting the microwaves and an applicator or resonator within which the microwaves are applied to the plastic preforms. The microwave generator usually has functional groups such as a rectifier and a high voltage cascade. This high voltage cascade generates a high voltage from DC voltage. Usually, first a mains voltage is rectified and then high voltage is in turn generated by means of said high voltage cascade. The necessary components, such as the generator, the magnetron, the waveguide and the applicator, each being of low power, are therefore installed in each individual station and are operated in parallel. A large number of functional groups is thus necessary, as a result of which on the one hand the costs are increased and on the other hand the results are sometimes unsatisfactory.